More than 90% of the alumina production of the world is prepared by the Bayer process, on applying a digestion temperature of 100.degree.-140.degree. C. and 200.degree.-250.degree. C., respectively, depending on the gibbsite or bohmite-diaspore nature of the processed bauxites. The predominant part of the bauxite reserve of the world consists of lateritic bauxites of gibbsite character that contain in general 3 to 20% of bohmite and eventually even 1 to 2% of diaspore whereas the predominant iron mineral is goethite. By isomorphous substitution a maximum of 33 mole % of aluminum may be built-in into the goethite lattice in the place of the Fe atoms. This may cause eventually decreases of up to 5-10% in the yield of alumina. In addition to aluminum, we found that vanadium can be built into the goethite lattice, and in this way this amount of vanadium does not get dissolved into the aluminate liquor under the conditions of the digestion.
In the karstic bauxites of the bohmite-gibbsite type, there is generally an appreciable amount of goethite beside the hematite. On processing the Hungarian, French and Yugoslav bauxites of this type the aluminum containing goethite mineral remains unchanged in the applied temperature range from 200.degree. to 250.degree. C. This causes on one hand an increase in loss of alumina, and on the other hand, owing to the less compact crystal structure of goethite, a reduction in the sedimentability, filterability and washability of the formed red muds. In case of these bauxites also the grade vanadium dissolution is lower too. It is known that the goethite content of the bauxites of Drnis and of Iszkaszentgyorgy can be converted into hematite in a tubular digestion device only at a temperature of 320.degree. to 340.degree. C.
In certain bauxites also lepidocrocite and maghemite occur. The latter contains as a rule a great amount of isomorphe substituted aluminum, and this diminishes similarly to alumogoethite the alumina yield and the separability of the red mud. Its behavior is also similar to that of goethite. Consequently, any reference in the specification and in the claims to goethite is intended also to include lepidocrocite and maghemite. This difficulty could be eliminated for a long time only by the roasting of bauxite.
It is known from recent communications of the literature that at the digestion of goethite-containing bauxites of this type containing isomorphe substituted aluminum the conversion of alumo-goethite into hematite occurs at lower temperatures when CaO is present. The rate and degree of conversion depends also on the period of digestion and on the composition of the aluminate liquor used for digestion. Thus the rate and degree of conversion can be influenced by the variation of these factors. However, the rise of the temperature of digestion, of the alkali concentration and of the molar ratio attained after digestion is restricted partly by availability of apparatus and partly by technological and economical limits. Thus, temperatures over 250.degree. C. cannot be maintained in series of autoclaves. The application of tube digesters of higher temperature still present unsolved technical difficulties, and the high-temperature variant of tube digesters still have not become widespread in industrial practice. The rise of the molar ratio after digestion, however, essentially decreases the efficiency of the Bayer cycle.